The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. A number of methods have been developed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from a subterranean formation containing oil shale has been described in several issued patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by reference. This patent describes the in situ recovery of liquid and gaseous carbonaceous products from subterranean formations containing oil shale by preparing an in situ oil shale retort in the subterranean formation. The retort is formed by excavating a production tunnel or drift in the subterranean formation, mining at least one void in the formation within the boundaries of the in situ oil shale retort being formed, and explosively expanding formation toward such a void to form a fragmented permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying the hot retorting gases used for converting kerogen contained in the oil shale, as described in the '423 patent, includes the establishment of a combustion zone in the retort and the movement of an oxygen supplying gaseous feed mixture into the combustion zone to advance the combustion zone through the retort. In the combustion zone, oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying gaseous feed mixture into the combustion zone, the combustion zone is advanced through the retort.
The combustion gas and the portion of the gaseous feed mixture which does not take part in the combustion process pass through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale particles in the retort on the advancing side of the retorting zone. The liquid carbonaceous products, together with water, are withdrawn from the bottom of the retort through the production drift. An off gas containing combustion gas generated in the combustion zone, product gas produced in the retorting zone, gas from carbonate decomposition, and gaseous feed mixture that does not take part in the combustion process is also withdrawn from the bottom of the retort through the production drift.
The off gas contains nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, methane and other hydrocarbons, and sulfur compounds such as hydrogen sulfide. Hydrogen sulfide is an extremely toxic gas with a toxicity greater than that of hydrogen cyanide. It also possesses a powerful, objectionable odor with a threshold for human smell of about 0.0003 ppm. In addition, carbon monoxide contained in off gas is toxic. For this reason it is desirable to isolate workers from the off gas. This has been difficult to accomplish because the production tunnel or drift is used for two functions. It is used for excavating formation to prepare new in situ oil shale retorts in a retort preparation region of the formation and at the same time it is used for withdrawing off gas from active retorts in a producing region of the formation.
Therefore, there is a need for a method for recovering liquid and gaseous products from oil shale in a subterranean formation which avoids the danger of exposing workers to the off gas.